Power supplies in the prior art include linear power supplies and switching power supplies. Linear power supplies control output voltage by controlling the voltage drop across a power transistor which is connected in series with a load. The power transistor is operated in its linear region and conducts current continuously.
Switching power supplies control output voltage by using a power transistor as a switch to provide a pulsed flow of current to a network of inductive and capacitive energy storage elements. These active elements smooth the switched current pulses into a continuous and regulated output voltage. The power transistor is usually operated either in a cutoff or saturated state at a duty cycle required by the voltage differential between the input and output voltages. Varying the duty cycle varies the regulated output voltage of the switching regulator.
Linear regulators offer a number of advantages, one of which is fast transient response. However, linear regulators suffer from the drawback of inefficiency. Power that is not consumed by the load is dissipated as heat. Switching regulators also offer a number of advantages, a primary one of which is high efficiency. Since the control element in a switching regulator is either off or saturated, there is very little power dissipation. Switching regulators, however, have the drawback of poor precision regulation. Switching regulators typically have a slow response to varying load conditions. While the switching regulator circuit response time may be optimized for a particular, non-varying output potential and range of load fluctuations, there remains a limiting minimum response time under which the switching regulation is unable to adequately respond. Adding additional capacitance to the switching regulator is not a feasible solution for maintaining dynamic voltage regulation. The number of large local capacitors that would be needed for maintaining dynamic regulation would be impractical because of size and cost restraints. Moreover, the response time of the switcher could be affected by the addition of such capacitance which would compound the problem.
The prior art alternatives mentioned above fail to provide a solution to the problem of dynamic regulation for the load demands of newer microprocessor systems. Modern processors tend to modulate their current draw very rapidly and severely. They experience the problem of change in current per unit time (Di/Dt) when there is either a sudden increase or decrease in current demand. Modern systems require higher performance through the usage of a greater number of transistors at higher operating speeds. Obviously, this increases the power dissipation requirements of these new systems. A large load demand may also require a large amount of current to power the system during a given period of time when such current supply is unavailable. Designers of modern processors are also shutting off units when they are not in use and are calling them back into play when they are needed in order to conserve power in the system. Thus, the load demand for these systems may drop suddenly, resulting in an excess of unwanted current which could lead to damaging components within the system.
Thus, a voltage regulator which is able to respond to the changing power requirements of a load and operate efficiently is needed. This regulator must be able to respond quickly both to increases and decreases in power demand. As will be seen, the present invention overcomes the drawbacks of the prior art by providing a voltage regulating apparatus and method which use a first voltage regulator which operates with high efficiently, a second voltage regulator which operates with fast transient response, a load component which sinks excess current at the load, and control circuitry for managing the operation of these three components.